Mycoplasma pneumoniae commonly causes respiratory tract infections in humans, but it may also be associated with central nervous system manifestations. The aim of the present study was to determine whether the cerebrospinal fluid taken from patients with neurologic symptoms due to Mycoplasma pneumoniae infection contains specific antibodies and whether the detection of these antibodies can be used for diagnosis. Mycoplasma pneumoniae was isolated from the cerebrospinal fluid taken from nine patients with central nervous system symptoms on admission to the hospital. In addition, Mycoplasma pneumoniae was detected in cerebrospinal fluid using polymerase chain reaction in four other patients. Antibodies to Mycoplasma pneumoniae were detected using the enzyme immunosorbent assay, indirect immunoperoxidase assay and immunoblotting in cerebrospinal fluid samples from 14 of 19 patients included in the study. The indirect immunoperoxidase assay showed high titers of Mycoplasma pneumoniae immunoglobulin G1 (IgG1) and IgM antibodies in cerebrospinal fluid samples of some patients with meningoencephalitis or meningitis. Titers of specific IgA, IgG2 and IgG3 antibodies were lower, while specific IgG4 was not detectable. Cerebrospinal fluid samples with higher antibody titers also contained IgA, IgG1, IgG2, IgG3 and IgM antibodies that recognized the P1 adhesin (170 kDa protein) of Mycoplasma pneumoniae. A comparison of antibody titers of concomitant serum/cerebrospinal fluid samples to Mycoplasma pneumoniae and those to measles virus by enzyme immunosorbent assay suggested the intrathecal synthesis of IgG and IgM antibodies to Mycoplasma pneumoniae in patients with acute meningoencephalitis. Data from this study clearly reinforce previous findings that Mycoplasma pneumoniae is an etiologic agent of central nervous system infections in humans.